soften shadows projected by HDRi
A small light source will create sharp shadows. In the HDRi image you are using, the direct sun will create a very defined shadow.



A different HDR, this time of a cloudy day, will create a much larger light source, and less defined shadows.



As an alternative you can add objects that will reflect some of the light into the shadowed areas:



Or maybe you don't need to use the HDR and just use a large source that will give you a soft shadow.



Or a radical approach is to blur the HDR image used as environment texture, making the light source less defined:



Note that you might need to change the strength used for the environment texture

(for info on how this works please watch Bartel Skorupa's video Manipulate texture coordinates like a boss)